1. Field of the Invention
The invention relates generally to drill bits that have polycrystalline diamond compact (xe2x80x9cPDCxe2x80x9d) cutters thereon.
2. Background Art
Polycrystalline diamond compact (xe2x80x9cPDCxe2x80x9d) cutters have been used in industrial applications including rock drilling and metal machining for many years. In these applications, a compact of polycrystalline diamond (or other superhard material such as cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide, to form a cutting structure. A compact is a polycrystalline mass of diamonds (typically synthetic) that is bonded together to form an integral, tough, high-strength mass.
A PDC cutter may be formed by placing a cemented carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and compressed under high pressure, high temperature conditions. In so doing, metal binder migrates from the substrate and passes through the diamond grains to promote a sintering of the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is subsequently bonded to the substrate. The substrate is often a metal-carbide composite material, such as sintered tungsten carbide (tungsten carbide/cobalt).
An example of a rock bit for earth formation drilling using PDC cutters is disclosed in U.S. Pat. No. 5,186,268. FIGS. 1 and 2 from that patent show a rotary drill bit having a bit body 10. The lower face of the bit body 10 is formed with a plurality of blades 16-25, which extend generally outwardly away from a central longitudinal axis of rotation 15 of the drill bit. A plurality of PDC cutters 26 is disposed side by side along the length of each blade. The number of PDC cutters 26 carried by each blade may vary. Each PDC cutter 26 is received and secured within a cutter pocket 27 in the respective blade and bonded to the pocket by brazing.
U.S. Pat. No. 4,453,605, issued to Short, Jr., describes a typical method of brazing. In a typical brazing technique, a braze foil (which may be a silver brazing alloy, for example) is wrapped around a cutter. The braze foil is wrapped on the cutter in a manner such that the excess foil is allowed to protrude above a top side of the cutter. Thus, when placed in a cutter pocket, the excess foil protrudes through a gap in the cutter pockets (i.e., the gap is a clearance between the cutter and the cutter pocket). The bit head is then heated to a predefined brazing temperature of on the order of 1300xc2x0 F., but less than a thermal degradation temperature of the cutter. The system is allowed to cool, causing the braze material to harden, which fixes the cutter in the cutter pocket. Cooling may be performed by passing an inert gas over the drill bit.
In one class of PDC bits, PDC cutters are brazed into the pockets of a matrix body. In these bits, the matrix material is typically infiltrated with binder materials to form the body. In another class of PDC bits, PDC cutters are brazed into the pockets of a machined steel body, commonly referred to as xe2x80x9csteel body PDC bits.xe2x80x9d While steel body bits have toughness and ductility properties which make them resistant to failure due to impact forces generated during drilling, steel is more susceptible to erosive wear caused by high-velocity drilling fluids and formation fluids which carry abrasive particles, such as sand and rock cuttings. Therefore, these steel bits are normally xe2x80x9chardfacedxe2x80x9d with a hard, wear resistant coating to provide additional abrasive and erosive resistance.
U.S. Pat. No. 6,196,338, issued to Slaughter, describes one typical method of depositing a hardfacing layer on a roller cone drill bit. Hardfacing material generally includes a metallic component and a nonmetallic component. The metallic component can be any metal or metal alloy, such as iron, steel, nickel-based alloys, etc. The nonmetallic component generally includes a hard material, such as carbide, boride, and/or nitride. The volume content of the carbide phase is generally in the range of about 25%-60%, depending on the particular requirements. Examples of the carbides used in the carbide phase include single crystal tungsten carbide, eutectic WC/W2C, sintered WC/Co, or a combination of the above. The balance of the hardfacing material is a binder matrix, which is generally in the range of about 35%-75% of the volume of the hardfacing material. Often, the hardfacing material is formed in the shape of a xe2x80x9ctube,xe2x80x9d which is then heated by a torch to deposit the hardfacing material onto the cutter surface. Such a method is often termed xe2x80x9ctube hardfacing.xe2x80x9d
Typically, the hardfacing is applied to the entire bit. A hardfacing layer may be deposited onto the steel surface by the torch method mentioned above, by thermal spray, or alternatively, the hardfacing material may be deposited by an xe2x80x9carc process.xe2x80x9d In particular, a plasma transferred arc (PTA) welding process may be used. The PTA welding process uses a torch similar to a conventional plasma arc torch with an electrode grounded to the work piece. The PTA system generally includes two power supplies: a pilot arc power supply and a transferred arc power supply. In the PTA welding process, a pilot plasma arc is initiated between a tungsten electrode and a copper orifice with a water cooled electrode. An inert gas, such as argon, flowing through the orifice is ionized so that it initiates a secondary arc between the tungsten electrode and the bit, when the current is increased. Hardfacing powder of a suitable composition is injected into the plasma column by a carrier gas. A molten pool forms on the bit in the arc transfer region. Fusion occurs between the deposited powder and the bit.
Typically, after a hardfacing layer has been applied to the bit body in a PDC bit, the PDC cutters are then brazed into the cutter pockets. However, as described above, a non-hardfaced region may exist between the PDC cutter and the cutter pocket, called a xe2x80x9cgap,xe2x80x9d herein. The gap is generally filled with the melted metal brazing alloy, which may be a silver brazing alloy. When the PDC bit is used to drill formations in a downhole environment, wellbore fluids may erode the braze material around the cutter. The braze alloy is relatively soft as compared to the other materials and offers little erosion resistance. If the erosion becomes significant, the steel surrounding the braze deposit also becomes eroded, and a deep cavity forms around the PDC cutters. The strength of the braze may be reduced such that the PDC cutter may be forced out of the cutter pocket, thereby altering a cutting structure and, e.g., a force distribution over the bit. If a significant number of PDC cutters are forced from their respective pockets, drilling operations may have to be stopped so that a new bit can be attached.
What is needed, therefore, is a way to reduce erosion associated with braze alloy around PDC cutter elements.
In one aspect, the present invention relates to a method of increasing the durability of a drill bit having a bit body having a hardfacing layer thereon, with at least one blade disposed thereon, at least one cutter pocket disposed on the blade, and at least one cutter disposed in the cutter pocket. The method includes brazing at least one cutter into a cutter pocket so that a brazing material disposed between at least one cutter pocket and at least one cutter forms an exposed surface, and overlaying the exposed surface with a hardfacing material.
In another aspect, the present invention relates to a drill bit including a bit body having at least one blade thereon, at least one cutter pocket disposed on the blade, at least one cutter disposed in the cutter pocket, a braze material disposed between the at least one cutter and the at least one cutter pocket, the braze material having an exposed surface, and an overlay coating applied proximate to the at least one cutter, wherein the overlay coating is adapted to cover the exposed surface of braze material proximate to the at least one cutter.
In another aspect, the present invention relates to a method of increasing a durability of a drill bit having a bit body with at least one blade disposed thereon, at least one cutter pocket disposed on the blade, and at least one cutter disposed in the cutter pocket, where the method includes brazing the at least one cutter to the at least one cutter pocket so that a braze material disposed between the at least one cutter pocket and the at least one cutter defines an exposed surface, and overlaying the exposed surface with a hardfacing material, the hardfacing material including a binder with sufficiently low melting point so as not to damage the at least one cutter.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.